Target detection systems, whether they be radar or sonar, view both noise and clutter in addition to the target. A typical source of clutter is the rough surface of an ocean surrounding a ship-borne radar. The surface of the ocean can also be a source of clutter to a sonar system. Raindrops can serve as a source of clutter in the situation wherein several sweeps of a radar occur within an interval of time which is short compared to the period of raindrop activity. Range gating is employed in such detection systems for dividing each range sweep into a set of range cells in which one or more targets may be detected. Target echo signals are compared to a threshold by a comparator to determine the presence of a target. Statistically, there is a rate at which detections are erroneously reported due to the presence of noise and clutter, the rate being known as a false alarm rate. Circuitry which adjusts the relative magnitudes of the signal to the threshold in inverse proportion with the mean value of the background noise to provide a constant false alarm (CFAR) are often referred to as CFAR systems or circuits. To increase the sensitivity of detection, detection systems typically employ sweep-to-sweep integration for each range cell. A CFAR circuit placed ahead of the integrator reduces the dynamic range requirements of the integrator, thereby simplifying the circuitry of the integrator.
In the ordinary CFAR circuit, the amplitude of the signal is divided by the mean value of a set of samples of the background noise to provide a constant false alarm rate. Alternatively, the mean value is applied to the reference terminal, or threshold, of the comparator as a scaling factor to provide the constant false alarm rate. Such circuits have been successfully employed, in the absence of clutter, since the noise shows statistical independence between successive range cells of a range as well as statistical independence between successive range sweeps. Thus, even with the aforementioned integration, there is generally no build-up of a false echo, in the statistical sense, since the noise tends to be cancelled by integration.
However, in the case of clutter, a problem has arisen. While clutter shows a noise-like statistical independence from cell to cell of a range sweep, the clutter does exhibit significant correlation from sweep to sweep and, in addition, may be characterized by a non-stationarity in the statistical sense. As a result, the aforementioned integration of the values of the signals in a range cell from sweep to sweep introduces a substantially larger signal value in the presence of clutter than in the absence of clutter. The larger signal from the clutter increases the false alarm rate in spite of the use of the conventional CFAR system. Thus, it is seen that the integrator of a radar or sonar detection system provides improved detection of targets in the absence of noise only, but offers relatively little improvement in the detection of a target in the presence of clutter.